1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method for reading images on a top face and a back face of a document.
2. Description of the Related Art
As an image reading apparatus, there is an image reading apparatus in which images on a top face and a back face of a document conveyed by an ADF (auto document feeder: automatic document conveyance apparatus) are simultaneously read by two reading sensors to transfer the read image data to a host computer. In the image reading apparatus (scanner) as described above, there may be a case where the rate at which the document image is read is higher than the rate at which the image data is transferred to the host computer. In this case, the read image data is temporarily accumulated in a buffer memory (storage unit) in the reading apparatus to sequentially transfer the accumulated image data to the host computer. When an excessive amount of image data is accumulated in the buffer memory, then the image reading processing is suspended. Then, after the buffer memory is emptied, the image reading processing is resumed. The suspension of the document reading processing as described above is called a SS processing (start-stop processing).
When the SS processing as described above is carried out, the continuity of the image reading processing may be lost, thus causing the read image to have a deteriorated image quality. When the SS processing is performed during the reading of a document image by one reading sensor of two reading sensors positioned in a document conveyance path, an approach may be used, as disclosed in Japanese Patent Laid-Open Publication No. S60-100871 (1985), to delay the image reading processing at the resumption of the reading processing. Specifically, when the document after the stoppage of the conveyance is conveyed again and the reading processing is resumed, the image data corresponding to a predetermined number of already-read lines can be processed so as to be prevented from being read again, thus maintaining the continuity of the image reading processing. Japanese Patent Laid-Open Publication No. H7-221928 (1995) discloses a configuration according to which, when the memory buffer is full, the SS processing is also performed when the two reading sensors are used to read the images on a top face and a back face of a document.
When the SS processing is performed during the reading by the two reading sensors of the images on a top face and a back face of a document, a document behavior may cause, during the resumption of the reading processing, a difference between the number of lines in the top face image and the number of lines in the back face image that should be processed so as to avoid an overlapped image data reading. On the other hand, a common correction unit may be included as a correction unit for correcting a read image by processing, at the resumption of the reading processing, the top face image data and the back face image data so as to avoid an overlapped reading of the respective pieces of data. The provision of the common correction unit is advantageous in simplifying the circuit configuration and the increase of the processing speed.
However, the provision of the common correction unit for the image data of the top face and the back face requires one number of lines to be processed by the correction unit. Thus, at least one of the images of the top face image and the back face image may be incorrectly connected, thus causing a risk of a read image having a deteriorated quality.